It's International Kissing Day. Let's talk about what's in a kiss... and what she wants.

Studies have shown that a woman discovers everything she needs to know about a potential lover from his very first kiss, and any guy who has tried to get to First Base on a date knows it. And it's not just humans who show affection this way. Bonobo chimpanzees kiss, elephants put their trunks in each other's mouths, foxes lick faces — all as signs of affection.

But why do humans kiss in the first place, and what's the reasoning behind it — both scientific, emotional, and chemical?

Kissing is different in various cultures, with lip-locks the modus operandi in only some parts of the world. Some societies rub noses with each other to show affection—Inuit (Eskimos), Polynesians and Malaysians still do—and others do pucker their lips to show they care. Although theories on kissing abound (one study says that social kissing originated with medieval knights as a way to find out if their wives had been drinking while they were away fighting), the most widely accepted scientific reason for kissing is that humans do it to find a suitable mate. When our faces are close together, our pheromones “communicate” primally—exchanging biological information about whether or not two people will make strong offspring.

From Scientific American, which has conducted more than a few studies on kissing:

A kiss triggers a cascade of neural messages and chemicals that transmit tactile sensations, sexual excitement, feelings of closeness, motivation and even euphoria.

Kissing may have evolved from primate mothers’ practice of chewing food for their young and then feeding them mouth-to-mouth. Some scientists theorize that kissing is crucial to the evolutionary process of mate selection.

A simple kiss is often the initial gauge by which one might judge the success of a relationship. According to scientists who make their careers studying kissing (they are called osculologists), there's an enormous amount of neurotransmitters, evolutionary biology, and instant assessments of potential life-mates.

More than half of all people experience their first real kiss by the time they are 14 years old, but it may not be a great experience for all. As adults, it might be a good idea to know more than a teenager about philematology (that is: the science/art of kissing):

Kissing is exercise. When we kiss, our hearts beat faster and our breathing becomes deep, mimicking exercise. Meaning that if done right, kissing can be a great workout, as a 60-second kiss burns more than 50 calories.

Kissing can be proper etiquette. In many European countries, it is proper etiquette to greet someone by kissing them on both cheeks.

Kissing raises self-esteem. Kissing signals our brain to produce hormones that makes us feel good. And it’s been proven that one kiss leads to another.

So, what’s the secret to a romantic kiss that will knock her proverbial socks off? After interviewing over 2,000 women on Facebook, here’s what women say they’re looking for in a kiss:

Commonly-discussed Don'ts:

"Sloppy, wet, and all over the place."

"His tongue all the way down my throat."

"Don't think that that a kiss is the precursor to me giving oral sex."

"Don't lick my mouth. That's nasty."

"What's up with fast jaw movements? I'm not trying to be chewed on."

The Please-Do'sStart gently. Let your mouth wander as she allows, but pay attention to her responses. Light strokes on cheeks, neck, and back get extra points, as these areas are usually ignored (at least initially). At the beginning, a light caress is definitely recommended, and should give way to a harder touch and grip as kissing continues. Timing is everything. Start slowly, and stay slow enough to watch for signs that encourage more rapid movement and advances. Kiss lips, cheeks, eyelids, and neck — slowly until she makes it clear she can't handle another second without... more.

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